CAMP
Professor Raymond Mackay (of Clarkson University's Department
of Chemistry) has been selected by the Maryland Section of the American
Chemical Society to receive the Maryland Chemist of the Year Award
for 2001. The award is in recognition of his outstanding contributions
to chemistry. Professor Mackay has numerous publications and many
prestigious honors. He has done extensive research in the areas
of micelles and the application of electrochemical methods. His
current research interests include the preparation of nanolatexes,
polymer/nanoparticle composites, and the effect of surfactants on
polishing slurries. He served as CAMP Director for eight years and
is currently on leave and serving as Acting Director of Research
and Technology at the U.S. Army Edgewood Chemical Biological Center,
Aberdeen Proving Ground in Maryland.

CAMP
Editor/Technical Writer Dr. Dana Barry received an APEX 2001
Award of Publication Excellence, for editorial content and over-all
communications excellence, for the 1999-2000 CAMP Annual Report
Newsletter. Designer Sharon Williams (of Ad Workshop in Lake
Placid, NY) received an APEX 2001 Award of Publication Excellence
for the design and layout of the same newsletter. Both awards were
given by Communications Concepts, Inc. in Springfield, VA.

THE
RESEARCH

MODELING
OF THE CHEMICAL-MECHANICAL POLISHING PROCESS

Professor Ahmadi and
his group are developing a model (based on mechanical contact theory)
for the chemical-mechanical polishing process. The goal of their research
is to provide a fundamental understanding of the parameters that control
the effectiveness of CMP for surface planarization. Their current work
focuses on the abrasive particle, wafer, and pad contact and the abrasive
and adhesive wear mechanisms in the chemical-mechanical polishing process.
They are developing a model for interactions of pad asperities with abrasive
particles and the wafer. Their analysis includes the influence of abrasive
particle adhesion to the surface of the wafer. Also they are looking at
the CMP process using hard and soft pads and dilute and concentrated slurries.
In addition Professor Ahmadi and his students are also studying the effect
of abrasive particle shapes, slurry pH, and colloidal forces on the removal
rate.

Their model predictions are described in detail and compared
with the available semi-empirical correlations in the paper " A Model
for Mechanical Wear and Abrasive Particle Adhesion During the Chemical-Mechanical
Polishing Process," by G. Ahmadi and X. Xia, Journal of the Electrochemical
Society , 148 (3) G99-G109 (2001).

Professor R. Shankar Subramanian
is working on various aspects of modeling of chemical- mechanical polishing.
He is interested in predicting overall removal rates from blanket wafers,
and phenomena such as dishing and erosion in patterned wafers. Along with
former graduate student Lu Zhang, he has developed a detailed transport
model in which the convective and diffusive transport of chemical species
in the pores of the polishing pad is accommodated using a two-dimensional
repeating cell description. The appropriate conservation equations for
momentum and species, along with the boundary conditions, are solved numerically
in this approach. The model can predict the chemical removal rate as a
function of relative speed between the wafer and the pad and geometric
parameters. Abrasive removal of material can be accommodated in the transport
model as well. He and graduate student Lu Zhang have recently used the
model to predict copper removal rates in a Strasbaugh 6CA polishing tool
by an abrasive-free solution containing hydrogen peroxide and glycine,
and compared these predictions with experimentally measured rates. The
model correctly predicts the trend of the removal rate plotted against
the relative velocity between the wafer and the polishing pad over a range
of glycine concentrations, displaying a non-linear dependence on the velocity.
The removal rate is small at low relative velocities and approaches an
asymptotic rate at larger values of the relative velocity that corresponds
approximately to chemical removal at the incoming glycine concentration.
An article based on this work will shortly appear in Thin Solid Films.

Professor Subramanian also is interested in the process
by which mechanical removal of material occurs at the microscopic level.
Here, the issues are the role of the mechanical properties such as the
relative hardness of the wafer, abrasive particle, and the pad, the role
of asperities on the pad, and the coupling of the chemistry to the mechanical
removal process. He and graduate student Lirong Guo have been studying
the mechanical removal of copper in an alumina slurry as a function of
relative velocity, applied pressure, and particle concentration using
a Struers Benchtop polisher. Experiments have been performed using both
IC-1000 and Suba 500 pads. The results clearly demonstrate the inadequacy
of the Preston model in describing mechanical removal rates over a wide
range of velocities and pressures

Graduate student Rajesh Appat is working with Professor
Subramanian on predicting the polishing rates of steps on patterned wafers.
The initial modeling work is focused on fixed abrasive pads used for polishing
oxide films in applications such as shallow trench isolation (STI). In
fixed abrasive pads, an abrasive such as ceria is incorporated into pillars
on a flat polymer substrate, and is held together by a binder. The binder
disintegrates during polishing, exposing the abrasive particles. Two important
reported features of fixed abrasive polishing of STI oxide films are that
the rate at which a step is polished is a stronger function of pattern
density than is the case when a slurry containing abrasive particles is
used, and that the polish rate of blanket films is extremely low. A preliminary
model has been developed that captures these two features, and an article
that contains information about the model and sample predictions is scheduled
for publication in the December 2001 issue of Electrochemical and Solid
State Letters.

Professor Subramanian has recently begun working with a
new doctoral student Qingjun Qin on developing theoretical descriptions
of various aspects of polishing in an orbital polishing tool. A SpeedFam/IPEC
676 orbital tool has recently been installed at Clarkson, and the predictions
from models to be developed in this project will be compared with experimental
data obtained from this tool.